U.S. patent application number 12/248625 was filed with the patent office on 2009-10-01 for honeycomb structural body.
This patent application is currently assigned to IBIDEN CO., LTD.. Invention is credited to Shinnosuke Goto, Masafumi KUNIEDA, Toshiyuki Miyashita.
Application Number | 20090247395 12/248625 |
Document ID | / |
Family ID | 39941528 |
Filed Date | 2009-10-01 |
United States Patent
Application |
20090247395 |
Kind Code |
A1 |
KUNIEDA; Masafumi ; et
al. |
October 1, 2009 |
HONEYCOMB STRUCTURAL BODY
Abstract
A honeycomb structural body includes at least one honeycomb unit
which has a longitudinal direction. The at least one honeycomb unit
includes plural cell walls, an inorganic binder, and inorganic
particles. The plural cell walls extends along the longitudinal
direction from one end face to another end face of the at least one
honeycomb unit to define plural cells. The inorganic particles
include ceria particles, and a ceramic material which has a degree
of self-sintering lower than a degree of self-sintering of the
ceria particles.
Inventors: |
KUNIEDA; Masafumi; (Ibi-Gun,
JP) ; Goto; Shinnosuke; (Ibi-Gun, JP) ;
Miyashita; Toshiyuki; (Ibi-Gun, JP) |
Correspondence
Address: |
DITTHAVONG MORI & STEINER, P.C.
918 Prince St.
Alexandria
VA
22314
US
|
Assignee: |
IBIDEN CO., LTD.
Ogaki-Shi
JP
|
Family ID: |
39941528 |
Appl. No.: |
12/248625 |
Filed: |
October 9, 2008 |
Current U.S.
Class: |
502/73 ; 502/263;
502/304 |
Current CPC
Class: |
B01J 23/63 20130101;
C04B 37/005 20130101; C04B 2237/341 20130101; C04B 2237/04
20130101; C04B 2235/5264 20130101; C04B 35/6303 20130101; Y02T
10/12 20130101; C04B 2237/34 20130101; C04B 2235/3217 20130101;
C04B 2235/5244 20130101; C04B 35/803 20130101; B01J 37/0201
20130101; C04B 35/44 20130101; C04B 2235/5228 20130101; C04B
2237/708 20130101; C04B 35/50 20130101; C04B 2235/349 20130101;
F01N 3/0222 20130101; C04B 35/16 20130101; C04B 2235/5436 20130101;
C04B 2235/5445 20130101; C04B 2235/5232 20130101; C04B 2235/526
20130101; C04B 2235/5224 20130101; C04B 2237/343 20130101; C04B
2111/0081 20130101; C04B 2235/3218 20130101; C04B 35/6316 20130101;
C04B 35/82 20130101; C04B 2235/3229 20130101; C04B 2235/322
20130101; C04B 2235/522 20130101; C04B 38/0006 20130101; C04B
2111/00793 20130101; C04B 38/0006 20130101; C04B 35/50 20130101;
C04B 38/0054 20130101; C04B 38/0074 20130101 |
Class at
Publication: |
502/73 ; 502/304;
502/263 |
International
Class: |
B01J 23/10 20060101
B01J023/10; B01J 21/16 20060101 B01J021/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2008 |
JP |
PCT/JP2008/055969 |
Claims
1. A honeycomb structural body comprising: at least one honeycomb
unit having a longitudinal direction and comprising: plural cell
walls extending along the longitudinal direction from one end face
to another end face of the at least one honeycomb unit to define
plural cells; an inorganic binder; and inorganic particles
comprising: ceria particles; and a ceramic material having a degree
of self-sintering lower than a degree of self-sintering of the
ceria particles.
2. The honeycomb structural body according to claim 1, wherein the
ceramic material comprises at least one of alumina, silica,
zeolite, a precursor of alumina, a precursor of silica, and a
precursor of zeolite.
3. The honeycomb structural body according to claim 1, wherein an
average particle size of the ceramic material is less than or equal
to an average particle size of the ceria particles.
4. The honeycomb structural body according to claim 1, wherein an
average particle size of the ceria particles is in a range of
approximately 0.1 .mu.m through approximately 10 .mu.m.
5. The honeycomb structural body according to claim 1, wherein a
weight ratio of the ceramic material to the ceria particles is in a
range of approximately 10% through approximately 40%.
6. The honeycomb structural body according to claims 1, wherein the
inorganic binder comprises at least one of alumina sol, silica sol,
titania sol, liquid glass, sepiolite, and attapulgite.
7. The honeycomb structural body according to claim 1, wherein the
at least one honeycomb unit further comprises inorganic fiber.
8. The honeycomb structural body according to claim 7, wherein the
inorganic fiber comprises at least one of alumina, silica, silicon
carbide, silica alumina, glass, potassium titanate, and aluminum
borate.
9. The honeycomb structural body according to claim 1, wherein an
amount of the inorganic particles included in the at least one
honeycomb unit is approximately 30 wt % through approximately 90 wt
%.
10. The honeycomb structural body according to claim 1, wherein an
amount of the inorganic binder included in the at least one
honeycomb unit is approximately 5 wt % through approximately 50 wt
% as solids content.
11. The honeycomb structural body according to claim 7, wherein a
total amount of the inorganic fiber included in the at least one
honeycomb unit is approximately 3 wt % through approximately 50 wt
%.
12. The honeycomb structural body according to claim 1, wherein a
cell density of the at least one honeycomb unit is approximately
15.5 cells/cm.sup.2 through approximately 186 cells/cm.sup.2.
13. The honeycomb structural body according to claim 1, wherein a
thickness of each of the cell walls is approximately 0.1 mm through
approximately 0.4 mm.
14. The honeycomb structural body according to claim 1, wherein the
at least one honeycomb unit has plural pillar-shaped honeycomb
units, and wherein the honeycomb structural body comprises adhesive
layers joining the plural pillar-shaped honeycomb units.
15. The honeycomb structural body according to claim 1, wherein the
honeycomb structural body is produced as a single body without
adhesive layers.
16. The honeycomb structural body according to claim 1, wherein the
cell walls include a catalyst.
17. The honeycomb structural body according to claim 16, wherein
the catalyst includes at least one of noble metal, alkali metal,
alkali-earth metal, a rare-earth element, and transition metal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority under 35 U.S.C.
.sctn.120 to PCT Application No. PCT/JP2008/055969, filed Mar. 27,
2008. The contents of this application are incorporated herein by
reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a honeycomb structural
body.
[0004] 2. Description of the Related Art
[0005] Conventionally, a honeycomb structural body is provided in
an exhaust gas treating apparatus used for treating HC, CO,
NO.sub.X and the like included in exhaust gas discharged from
automobiles. This honeycomb structural body has plural cells
(through holes) which extend, along a longitudinal direction, from
one end face to the other end face of the honeycomb structural
body. These cells are separated from one another by cell walls.
[0006] For example, when the honeycomb structural body is used as a
catalyst carrier, the cell walls support a catalyst such as
platinum. When exhaust gas passes through such a catalyst carrier,
components such as HC, CO, NO.sub.X and the like included in the
exhaust gas are converted by catalyst reactions and the like,
because the cell walls support the catalyst. Accordingly, the
exhaust gas can be treated.
[0007] Such a honeycomb structural body is manufactured as follows.
First, pillar-shaped honeycomb units having the same shape are
joined together by interposing adhesive layers between the side
faces of the honeycomb units. Accordingly, a predetermined number
of honeycomb units are joined together. Then, the periphery of this
assembly of honeycomb units is cut into a desired shape.
[0008] The honeycomb units primarily include inorganic particles,
inorganic fiber, and an inorganic binder. There is a proposal for
using honeycomb units including inorganic particles which is
alumina or ceria (see, for example, WO 2005/063653).
[0009] Contents of WO 2005/063653 are incorporated herein by
reference in their entirety.
SUMMARY OF THE INVENTION
[0010] According to one aspect of the present invention, a
honeycomb structural body includes at least one honeycomb unit
which has a longitudinal direction. The at least one honeycomb unit
includes plural cell walls, an inorganic binder, and inorganic
particles. The plural cell walls extends along the longitudinal
direction from one end face to another end face of the at least one
honeycomb unit to define plural cells. The inorganic particles
include ceria particles, and a ceramic material which has a degree
of self-sintering lower than a degree of self-sintering of the
ceria particles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Other objects, features and advantages of the present
invention will become more apparent from the following detailed
description when read in conjunction with the accompanying
drawings, in which:
[0012] FIG. 1 is a schematic perspective view of an example of a
honeycomb structural body according to an embodiment of the present
invention;
[0013] FIG. 2 is a schematic perspective view of an example of a
honeycomb unit included in the honeycomb structural body shown in
FIG. 1; and
[0014] FIG. 3 is a perspective view of a honeycomb structural body
according to an embodiment of the present invention that is
different from the honeycomb structural body shown in FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] A description is given, with reference to the accompanying
drawings, of an embodiment of the present invention.
[0016] FIG. 1 is a schematic diagram of a honeycomb structural body
according to an embodiment of the present invention. FIG. 2 is a
schematic diagram of an example of a honeycomb unit, which is the
basic unit of the honeycomb structural body shown in FIG. 1.
[0017] As shown in FIG. 1, a honeycomb structural body 100
according to an embodiment of the present invention has two opening
faces 110 and 115. The peripheral surface of the honeycomb
structural body 100 is provided with a coat layer 120, except for
the two end faces.
[0018] The following describes an example of how the honeycomb
structural body 100 is formed. FIG. 2 illustrates a pillar-shaped
honeycomb unit 130 made of ceramics. Plural honeycomb units 130
(the example shown in FIG. 1 includes four horizontal rows and four
vertical rows, i.e., a total of 16 honeycomb units) are joined
together by interposing adhesive layers 150. Then the periphery is
cut into a predetermined shape (a cylindrical shape).
[0019] As shown in FIG. 2, the honeycomb unit 130 has plural cells
(through holes) 121 that are open at both end faces, and that
extend in a longitudinal direction from one end to the other end of
the honeycomb unit 130. Furthermore, the cells 121 are partitioned
by cell walls 123.
[0020] When the honeycomb structural body is used as a catalyst
carrier, the cell walls 123 support a catalyst such as platinum.
When exhaust gas flows into one of the cells 121, components (HC,
CO, NO.sub.X and the like) included in the exhaust gas are
converted, and then the exhaust gas is discharged from the same
cell. Accordingly, by making the exhaust gas flow through the
honeycomb structural body 100, the components in the exhaust gas
can be treated.
[0021] In the honeycomb structural body described in WO
2005/063653, different honeycomb units are used depending on the
purpose. Alumina is primarily used for high dispersion of noble
metal such as platinum, and ceria is primarily used for adhering
NO.sub.X.
[0022] However, honeycomb units primarily including ceria particles
are not as strong as honeycomb units primarily including alumina
particles. Thus, if a honeycomb structural body is to include
honeycomb units primarily including ceria particles, this honeycomb
structural body will become relatively easily breakable when
external stress is applied.
[0023] An embodiment of the present invention provides a honeycomb
structural body including ceria particles as inorganic particles,
and has a preferable level of strength.
[0024] The honeycomb unit includes inorganic particles and an
inorganic binder, and may also include inorganic fiber. Alumina
particles or ceria particles can be used as the inorganic particles
depending on the purpose. Generally, when molding a honeycomb, the
inorganic particles are used as secondary particles. However,
honeycomb units including ceria particles as inorganic particles
are not as strong as honeycomb units including alumina particles as
inorganic particles. Thus, if a honeycomb structural body is to
constitute honeycomb units primarily including ceria particles,
this honeycomb structural body will break relatively easily when
external stress is applied.
[0025] This is because ceria particles have a "high degree of
self-sintering". A term "high degree of self-sintering" means that
primary particles easily bind to each other, but secondary
particles do not easily bind to each other. Therefore, with
inorganic particles that have a "high degree of self-sintering", it
is difficult to achieve a fired body having a preferable level of
strength.
[0026] The degree of self-sintering of ceria particles is
considered to be higher than that of other inorganic materials such
as alumina due to the following reasons. Generally, when particles
bind together by sintering, the hydroxyl adhering to the surfaces
of the particles has an important function. That is, it is
considered that when hydroxyl is interposed between the particles,
cohesion between the particles is enhanced. However, it is
considered that in the case of ceria particles, the hydroxyl
adhering to the surfaces of ceria particles is not used for
cohesion between other substances, but for replenishing deficient
oxygen on the surfaces of the ceria particles. It is considered
that this is because ceria particles have more structural (lattice)
defects (for example, holes) than other inorganic particles. Thus,
it is considered that in the case of ceria, the amount of hydroxyl
used for cohesion between other particles is relatively small.
[0027] Ceria has a high level of self-sintering, and therefore if a
honeycomb unit is to include ceria particles, it will be difficult
to achieve a honeycomb unit having a preferable level of strength
by performing molding and firing processes. Furthermore, such a
honeycomb unit including ceria particles is fragile and particles
easily fall out. Thus, it is difficult to directly use ceria as the
material of a honeycomb unit.
[0028] However, a honeycomb structural body according to an
embodiment of the present invention has a preferable level of
strength, even though the honeycomb structural body constitutes
honeycomb units including ceria as the inorganic particles.
[0029] This is because the honeycomb units included in the
honeycomb structural body according to an embodiment of the present
invention includes inorganic particles other than ceria particles,
i.e., a ceramic material other than ceria particles having a lower
degree of self-sintering than ceria particles. That is, the
honeycomb unit according to an embodiment of the present invention
includes ceria particles, a ceramic material other than ceria
particles having a lower degree of self-sintering than ceria
particles, and an inorganic binder. The ceramic material other than
ceria particles having a lower degree of self-sintering than ceria
particles is not limited to one kind of material, but may include
two or more kinds of materials. For example, the ceramic material
other than ceria particles having a lower degree of self-sintering
than ceria particles may be alumina, silica, zeolite, boehmite, or
the like.
[0030] By adding, as inorganic particles, such a ceramic material
other than ceria particles having a lower degree of self-sintering
than ceria particles to the honeycomb unit, it will be easier to
achieve a honeycomb unit having a preferable level of strength. It
is considered that this is because the ceramic material other than
ceria particles having a lower degree of self-sintering than ceria
particles has a function of enhancing the cohesion between the
ceria secondary particles.
[0031] The "degree of self-sintering" of the inorganic particles
can be determined from the behavior of how the specific surface of
a member including the subject inorganic particles changes after
heat treatment. This is because if a member includes inorganic
particles having a high "degree of self-sintering", its specific
surface tends to decrease significantly.
[0032] For example, Table 1 shows how the specific surface changed
after heat treatment under various conditions, for samples
including different kinds of inorganic particles. In each
experiment, the specific surface of the sample was measured by a
so-called BET-N2 method (measuring apparatus: automatic specific
surface and pore distribution measuring apparatus (TriStar 3000)
manufactured by Shimadzu Corporation). Each sample was turned into
powder weighing 0.2 g before being measured. As a result, it was
found that in the sample including ceria particles, the specific
surfaces decreased more significantly than the other samples
(including .gamma. alumina or zeolite). This is because the degree
of self-sintering of ceria particles is higher than that of the
other inorganic particles. As shown in these results, the degree of
self-sintering increases in the order of zeolite, .gamma. alumina,
and ceria.
TABLE-US-00001 TABLE 1 SPECIFIC SURFACE (m.sup.2/g) AFTER HEAT
TREATMENT AFTER 5 AFTER 5 AFTER 5 AFTER 5 AFTER 5 INORGANIC BEFORE
HOURS IN HOURS IN HOURS IN HOURS IN HOURS IN PARTICLE TREATMENT
500.degree. C. 600.degree. C. 700.degree. C. 800.degree. C.
900.degree. C. .gamma. ALUMINA 194 188 185 182 171 150 ZEOLITE 377
373 372 364 349 342 CERIA 155 145 109 70 52 28
[0033] As described above, the honeycomb unit according to an
embodiment of the present invention constitutes, as inorganic
particles, ceria particles and a ceramic material other than ceria
particles having a lower degree of self-sintering than ceria
particles. Thus, in such a honeycomb unit, the cohesion between
inorganic particles is enhanced, therefore it is easy to achieve a
honeycomb unit and moreover a honeycomb structural body having a
preferable level of strength. Furthermore, by using ceria particles
and a ceramic material other than ceria particles having a lower
degree of self-sintering than ceria particles as inorganic
particles, it will be easy to significantly reduce the amount of
particles falling out from the finished honeycomb unit.
[0034] In an embodiment of the present invention, the ceramic
material having a lower degree of self-sintering than ceria
particles, which is used as the inorganic particles other than
ceria particles, includes alumina, silica, zeolite, and
boehmite.
[0035] Furthermore, the average particle size of the ceramic
material other than ceria particles having a lower degree of
self-sintering than ceria particles, is preferably approximately
less than or equal to the average particle size of the ceria
particles. Moreover, the average particle size of the ceria
particles is preferably within a range of approximately 0.1 .mu.m
through approximately 10 .mu.m.
[0036] The weight percentage of the ceramic material other than
ceria particles having a lower degree of self-sintering than ceria
particles with respect to the ceria particles is preferably within
a range of approximately 10% through approximately 40%.
[0037] In the above description, the honeycomb unit 130 may include
an inorganic binder other than inorganic particles. In addition,
the honeycomb unit 130 can include inorganic fiber.
[0038] As the inorganic binder, inorganic sol, a clay-based binder
or the like can be used. Specific examples of the inorganic sol are
alumina sol, silica sol, titania sol, liquid glass, or the like. An
example of a clay-based binder is double-chain structure clay such
as white clay, kaoline, montmorillonite, sepiolite, attapulgite, or
the like. These can be used alone or in combination.
[0039] Among these, it is preferable to include at least one kind
selected from the group consisting of alumina sol, silica sol,
titania sol, liquid glass, sepiolite, and attapulgite.
[0040] Furthermore, if inorganic fiber were to be added to the
honeycomb unit, a preferable inorganic fiber material may be
alumina, silica, silicon carbide, silica alumina, glass, potassium
titanate, aluminum borate, or the like. These can be used alone or
in combination. Among these, alumina fiber is the preferable
material.
[0041] The lower limit of the amount of inorganic particles
included in a honeycomb unit is preferably approximately 30 wt %,
more preferably approximately 40 wt %, and even more preferably
approximately 50 wt %. Meanwhile, the preferable upper limit is
approximately 90 wt %, more preferably approximately 80 wt %, and
even more preferably approximately 75 wt %. If the content of
inorganic particles is more than or equal to approximately 30 wt %,
the relative amount of inorganic particles that can contribute to
converting NO.sub.X will hardly become small. Meanwhile, if the
content of inorganic particles is less than or equal to
approximately 90 wt %, the strength of the honeycomb unit will
hardly be reduced.
[0042] The amount of the inorganic binder included in a honeycomb
unit as solids content is preferably more than or equal to
approximately 5 wt %, more preferably more than or equal to
approximately 10 wt %, and even more preferably more than or equal
to approximately 15 wt %. Meanwhile, the amount of the inorganic
binder included in a honeycomb unit as solids content is preferably
less than or equal to approximately 50 wt %, more preferably less
than or equal to approximately 40 wt %, and even more preferably
less than or equal to approximately 35 wt %. If the content of the
inorganic binder as solids content was more than or equal to
approximately 5 wt %, the strength of the honeycomb unit would
hardly be reduced. Meanwhile, if the content of the inorganic
binder as solids content was less than or equal to approximately 50
wt %, the moldability would hardly be degraded.
[0043] When the honeycomb unit includes inorganic fiber, as to the
total amount of the inorganic fiber, the lower limit is preferably
approximately 3 wt %, more preferably approximately 5 wt %, and
even more preferably approximately 8 wt %. The upper limit is
preferably approximately 50 wt %, more preferably approximately 40
wt %, and even more preferably approximately 30 wt %. If the
content of the inorganic fiber was more than or equal to
approximately 3 wt %, the effect of the inorganic fiber on
enhancing the strength of the honeycomb unit would hardly be
reduced. If the content of the inorganic fiber was less than or
equal to approximately 50 wt %, the relative amount of inorganic
particles that can contribute to converting NO.sub.X would hardly
become small.
[0044] The cross-sectional shape of the honeycomb unit 130
perpendicular with respect to the longitudinal direction is not
particularly limited. It can be any shape as long as honeycomb
units can be joined together by interposing adhesive layers. The
shape of the honeycomb unit 130 cross section can be a square, a
rectangle, a hexagon, a sector, or the like.
[0045] Furthermore, the cross-sectional shape of each cell 121 of
the honeycomb unit 130 perpendicular with respect to the
longitudinal direction is not particularly limited. The shape can
be, for example, a triangle or a polygon other than a square.
[0046] The cell density of the honeycomb unit 130 is preferably
within a range of approximately 15.5 cells/cm.sup.2 through
approximately 186 cells/cm.sup.2 (approximately 100 cpsi through
approximately 1,200 cpsi), more preferably within a range of
approximately 46.5 cells/cm.sup.2 through approximately 170
cells/cm.sup.2 (approximately 300 cpsi through approximately 1,100
cpsi), and even more preferably within a range of approximately
62.0 cells/cm.sup.2 through approximately 155 cells/cm.sup.2
(approximately 400 cpsi through approximately 1,000 cpsi).
[0047] The thickness of the cell walls 123 of the honeycomb unit
130 is not particularly limited; however, in consideration of the
strength, the preferable lower limit is approximately 0.1 mm and
the preferable upper limited is approximately 0.4 mm.
[0048] As described above, the cell walls of such a honeycomb unit
support a catalyst such as noble metal. Examples of the noble metal
are platinum, palladium, rhodium, or the like, although not
particularly limited thereto.
[0049] The honeycomb structural body 100 according to an embodiment
of the present invention can be any shape. Other than the
cylindrical shape shown in FIG. 1, the shape of the honeycomb
structural body 100 can be, for example, a cylindroid, a square
pillar, a polygonal pillar, or the like.
[0050] The paste that forms the coat layer 120 and the adhesive
layers 150 of the honeycomb structural body 100 (paste for coat
layer and paste for adhesive layers) includes, for example,
inorganic particles and an inorganic binder, and may additionally
include inorganic fiber.
[0051] The coat layer 120 usually includes an inorganic binder and
inorganic fiber, and is also made from a raw material paste
including an organic binder. The organic binder may be, for
example, polyvinyl alcohol, methylcellulose, ethyl cellulose,
carboxyl methylcellulose, or the like. These can be used alone or
in combination. Among these organic binders, carboxyl
methylcellulose is preferable.
[0052] Then, the coat layer paste is applied to the peripheral
surface of the honeycomb structural body, and is dried and
solidified, thereby forming the coat layer. In the paste used as
the raw material, a pore forming material may be added according to
need, such as balloons that are microscopic hollow spheres made
from oxide-based ceramics, spherical acrylic particles, graphite,
or the like. The thickness of the coat layer is preferably
approximately 0.1 mm through approximately 2.0 mm.
[0053] In the honeycomb structural body 100 according to an
embodiment of the present invention, the adhesive layers 150 are
made from the same material as the coat layer 120. However, the
adhesive layers 150 may be made form a material different from that
of the coat layer 120.
[0054] The honeycomb structural body 100 can be applied to, for
example, an exhaust gas treating apparatus for treating exhaust gas
discharged from a diesel engine or the like.
[0055] In the example described above, the honeycomb structural
body is formed by joining together plural honeycomb units 130 by
interposing adhesive layers 150, as shown in FIG. 1. However, an
embodiment of the present invention is also applicable to a
honeycomb structural body manufactured by molding a single unit,
without the adhesive layers 150.
[0056] Such a honeycomb structural body 200 is shown in FIG. 3. As
shown in FIG. 3, the basic structure of the honeycomb structural
body 200 is the same as that of the honeycomb structural body 100
shown in FIG. 1. Therefore, in FIG. 3, elements corresponding to
those of the honeycomb structural body 100 are denoted by the same
reference numbers. The peripheral surface of the honeycomb
structural body 200 may be provided with or without a coat layer
similar to that of the honeycomb structural body 100.
[0057] It is obvious to those skilled in the art that the same
effects as described above can be achieved by including ceria
particles and a ceramic material other than ceria particles having
a lower degree of self-sintering than ceria particles in the
honeycomb structural body 200, at least in cell walls 124 of the
honeycomb structural body.
[0058] (Method of Manufacturing Honeycomb Structural Body)
[0059] Next, a description is given of a method of manufacturing a
honeycomb structural body according to an embodiment of the present
invention. The following describes an example of a method of
manufacturing particularly a honeycomb structural body formed with
plural honeycomb units.
[0060] First, a molded body of the honeycomb unit is manufactured
by extruding the molded body from a mold (die), out of a raw
material paste primarily including inorganic particles, inorganic
fiber, and an inorganic binder. As described above, the inorganic
particles include ceria particles and a ceramic material other than
ceria particles having a lower degree of self-sintering than ceria
particles. Other than these materials, in the raw material paste,
an organic binder, a dispersion medium, and a molding aid may be
added according to the moldability. As the organic binder, one or
more organic binders may be selected from methylcellulose, carboxyl
methylcellulose, hydroxyethylcellulose, polyethylene glycol,
phenolic plastic, epoxy resin, or the like, although not
particularly limited thereto. The blending quantity of the organic
binder is preferably approximately 1 part by weight through
approximately 10 parts by weight with respect to a total of 100
parts by weight of ceramic particles, inorganic fiber, and the
inorganic binder. Examples of the dispersion medium are water, an
organic solvent (e.g., benzene), alcohol (methanol) and the like,
although not particularly limited thereto. Examples of the molding
aid are ethylene glycol, dextrin, fatty acid, fatty acid soap,
polyalcohol and the like, although not particularly limited
thereto.
[0061] The raw material paste is preferably mixed and kneaded, for
example, by using a mixer, an attritor and the like to mix it and
by using a kneader or the like to sufficiently knead it, although
not particularly limited thereto. For example, an extrusion molding
method is a preferable method for molding the raw material paste
into a shape having cells, although the method is not particularly
limited thereto.
[0062] Next, the honeycomb molded body that has been formed is
preferably dried. Examples of a drying apparatus used for the
drying process are a microwave drying apparatus, a hot air drying
apparatus, a dielectric drying apparatus, a suction drying
apparatus, a vacuum drying apparatus, a freeze drying apparatus and
the like, although not particularly limited thereto. The resultant
honeycomb molded body is preferably degreased. The degreasing
conditions are preferably approximately 400.degree. C. for
approximately two hours, although these conditions are not
particularly limited thereto; the conditions are to be
appropriately selected depending on the kind and amount of the
organic substance included in the molded body. Furthermore, the
resultant honeycomb molded body is preferably fired. The firing
conditions are preferably approximately 600.degree. C. through
approximately 1,200.degree. C., more preferably approximately
600.degree. C. through approximately 1,000.degree. C., although not
particularly limited thereto. This is because if the firing
temperature was approximately more than or equal to 600.degree. C.,
the sintering of the inorganic particles would easily progress, and
therefore the honeycomb unit would hardly have a low level of
strength; if the firing temperature is less than or equal to
approximately 1,200.degree. C., the self-sintering of the ceria
particles would hardly progress excessively, and therefore the
adsorbing function of the NO.sub.X would hardly be degraded. By
performing these procedures, it is possible to form a honeycomb
unit having plural cells.
[0063] Next, a paste which later becomes the adhesive layers is
applied onto the side surfaces of the honeycomb units, in such a
manner as to have uniformity thicknesses. Sequentially, other
honeycomb units are stacked onto each other with adhesive layers
interposed therebetween. This procedure is repeated to manufacture
a honeycomb structural body of a desired size (for example, with
four horizontal rows and four vertical rows of honeycomb
units).
[0064] As the paste for adhesive layers, it is possible to use a
mixture of an inorganic binder and ceramic particles, a mixture of
an inorganic binder and inorganic fiber, a mixture of an inorganic
binder, ceramic particles and inorganic fiber or the like, although
not particularly limited thereto. It is also possible to add an
organic binder to this adhesive paste. As an example of the organic
binder, one or more kinds of organic binder may be selected from
polyvinyl alcohol, methylcellulose, ethyl cellulose, carboxyl
methylcellulose, or the like, although not particularly limited
thereto.
[0065] The thickness of the adhesive layer for joining the
honeycomb units is preferably approximately 0.3 mm through
approximately 2 mm. If the thickness of the adhesive layer was more
than or equal to approximately 0.3 mm, sufficient bonding strength
would be easily achieved. If the thickness was less than or equal
to approximately 2 mm, the pressure loss would hardly be large.
[0066] Next, the honeycomb structural body is heated to dry and
solidify the paste for adhesive layers, thereby forming the
adhesive layers and fixing together the honeycomb units.
Subsequently, this joined body is cut into a cylindrical shape.
[0067] Furthermore, it is possible to form a coat layer by applying
a paste for the coat layer onto the peripheral surface of the
honeycomb structural body, and then drying and solidifying the
paste. The paste for the coat layer can be the same as or different
from the paste for adhesive layers, although not particularly
limited thereto. The compounding ratio of the paste for the coat
layer can be the same as or different from that of the paste for
adhesive layers. The thickness of the coat layer is not
particularly limited.
[0068] After joining plural honeycomb units together with adhesive
layers (in the case of providing a coat layer, after forming the
coat layer), the honeycomb structural body is preferably
heat-treated. When an organic binder is included in the paste for
the adhesive layers and in the paste for the coat layer, the
organic binder is preferably degreased and removed by the heat
treatment. The conditions for the heat treatment are preferably
approximately 700.degree. C. for approximately two hours, although
these conditions may be changed depending on the kind and amount of
the included organic substance.
[0069] When a catalyst is supported on the cell walls of the
honeycomb structural body, examples of the catalyst material are
noble metals such as platinum, palladium, rhodium, or the like,
although not particularly limited thereto. Furthermore, the cell
walls may support a compound including an alkali metal, an
alkali-earth metal, a rare-earth element, a transition metal, or
the like. An example of a method of providing a platinum catalyst
is to impregnate the honeycomb unit provided with catalyst
supporting layers in dinitrodiammine platinum nitric acid solution
([Pt(NH.sub.3).sub.2(NO.sub.2).sub.2]HNO.sub.3) or the like, and
then heating the honeycomb unit.
EXAMPLES
[0070] Next, a description is given of examples according to the
embodiment of the present invention.
Example 1
[0071] First, 2,707 parts by weight of ceria particles (average
particle size 2 .mu.m), 289 parts by weight of alumina particles
(average particle size 0.5 .mu.m) as the ceramic material having a
lower degree of self-sintering than ceria particles, 345 parts by
weight of alumina fiber (average fiber diameter 6 .mu.m and average
fiber length 100 .mu.m), and 2,200 parts by weight of alumina sol
were mixed together. To the resultant mixture, 320 parts by weight
of methylcellulose as an organic binder, and small amounts of a
plasticizer, a surface-active agent, and a lubricant were added.
The mixture was mixed and kneaded to obtain a mixed compound. Next,
a raw molded body was extruded from the mixed compound by an
extrusion molding apparatus.
[0072] Next, a microwave drying apparatus and a hot air drying
apparatus were used to thoroughly dry the raw molded body. Then,
the raw molded body was degreased at 400.degree. C. for two hours.
Then, the raw molded body was fired at 700.degree. C. for two
hours, thereby achieving a porous honeycomb unit shaped as a square
pillar (size: height 35 mm.times.width 35 mm.times.length 150 mm).
The cell density of this porous honeycomb unit was 93
cells/cm.sup.2, and the cell wall thickness was 0.2 mm.
Example 2
[0073] Next, by the same method as that of example 1, a honeycomb
unit according to example 2 was manufactured. However, in example
2, the blending quantities of ceria and alumina were 2,418 parts by
weight and 577 parts by weight, respectively. The other conditions
were the same as those of example 1.
Example 3
[0074] Next, by the same method as that of example 1, a honeycomb
unit according to example 3 was manufactured. However, in example
3, the blending quantities of ceria and alumina were 2,130 parts by
weight and 866 parts by weight, respectively. The other conditions
were the same as those of example 1.
Example 4
[0075] Next, by the same method as that of example 1, a honeycomb
unit according to example 4 was manufactured. However, in example
4, boehmite was used as a ceramic material having a lower degree of
self-sintering than ceria particles. The blending quantities of
ceria and boehmite were 2,418 parts by weight and 577 parts by
weight, respectively. The other conditions were the same as those
of example 1.
Example 5
[0076] Next, by the same method as that of example 1, a honeycomb
unit according to example 5 was manufactured. However, in example
5, silica (average particle size 0.5 .mu.m) was used as a ceramic
material having a lower degree of self-sintering than ceria
particles. The blending quantities of ceria and silica were 2,418
parts by weight and 577 parts by weight, respectively. The other
conditions were the same as those of example 1.
Example 6
[0077] Next, by the same method as that of example 1, a honeycomb
unit according to example 6 was manufactured. However, in example
6, the alumina particles used as a ceramic material having a lower
degree of self-sintering than ceria particles had an average
particle size of 2 .mu.m. The other conditions were the same as
those of example 1.
Example 7
[0078] Next, by the same method as that of example 1, a honeycomb
unit according to example 7 was manufactured. However, in example
7, ceria having an average particle size of 10 .mu.m was used. The
blending quantities of ceria and alumina were 2,418 parts by weight
and 577 parts by weight, respectively. The other conditions were
the same as those of example 1.
Example 8
[0079] Next, by the same method as that of example 1, a honeycomb
unit according to example 8 was manufactured. However, in example
8, ceria having an average particle size of 0.1 .mu.m was used. The
blending quantities of ceria and alumina were 2,418 parts by weight
and 577 parts by weight, respectively. The other conditions were
the same as those of example 1.
Comparative Example 1
[0080] Next, by the same method as that of example 1, a honeycomb
unit according to comparative example 1 was manufactured. However,
in comparative example 1, only ceria having an average particle
size of 2 .mu.m was used as the inorganic particles. The other
conditions were the same as those of example 1.
[0081] In Table 2, the blending quantities of inorganic particles
as well as the average particle sizes of the inorganic particles
included in the honeycomb units according to the each of the
examples and the comparative example are shown together.
TABLE-US-00002 TABLE 2 PERCENTAGE (%) OF CERAMIC MATERIAL HAVING
LOW DEGREE OF CERAMIC MATERIAL HAVING LOW SELF-SINTERING WITH
DEGREE OF SELF-SINTERING RESPECT TO INORGANIC CERIA AVERAGE
PARTICLES (CERIA + COMPOSITION AVERAGE COMPOSITION PARTICLE CERAMIC
MATERIAL BEND (PARTS BY PARTICLE SIZE (PARTS BY SIZE HAVING LOW
DEGREE OF STRENGTH WEIGHT) (.mu.m) MATERIAL WEIGHT) (.mu.m)
SELF-SINTERING) (MPa) EXAMPLE 1 2707 2 ALUMINA 289 0.5 10.7 6.2
EXAMPLE 2 2418 2 ALUMINA 577 0.5 23.9 6.7 EXAMPLE 3 2130 2 ALUMINA
866 0.5 40.6 7.6 EXAMPLE 4 2418 2 BOEHMITE 577 -- 23.9 8.2 EXAMPLE
5 2418 2 SILICA 577 0.5 23.9 5.3 EXAMPLE 6 2418 2 ALUMINA 577 2
23.9 5.1 EXAMPLE 7 2418 10 ALUMINA 577 0.5 23.9 6.3 EXAMPLE 8 2418
0.1 ALUMINA 577 0.5 23.9 6.7 COMPARATIVE 2995 2 -- -- -- -- 3.3
EXAMPLE 1
[0082] (Evaluation of Strength of Honeycomb Units)
[0083] Strength evaluation tests were performed on the honeycomb
units according to examples 1 through 8 and comparative example 1
manufactured by the above methods. The strength evaluation test was
conducted by performing three-point bend measurement. The
measurement was performed in accordance with JIS-R1601, with the
use of a three-point bend test apparatus 5582 manufactured by
Instron. The contents of JIS-R1601 are incorporated herein by
reference in their entirety.
[0084] The measurement was performed as follows. First, the
crosshead speed was set at 1 mm/minute, and the inter-span length L
was set at 135 mm. The breaking weight W was measured for each
honeycomb unit, by applying weight in the vertical direction with
respect to the longitudinal direction of each honeycomb unit. Next,
the moment of the part of the cells of the honeycomb unit was
subtracted to obtain a cross-sectional two dimensional moment Z.
Then, the three-point bend strength .sigma. was calculated with the
following formula.
.sigma.=WL/4Z formula (1)
[0085] The measurement results for each of the honeycomb units are
shown together in Table 2. It was confirmed with these results that
the bend strengths of the honeycomb units according to examples 1
through 8 were significantly higher than that of the honeycomb unit
according to comparative example 1.
[0086] The present invention is not limited to the specifically
disclosed embodiment, and variations and modifications may be made
without departing from the scope of the present invention.
* * * * *